What is Subsidence?

Subsidence is a term that strikes fear into the hearts of property owners and potential buyers alike. It refers to the downward movement of the ground supporting a building, which can lead to structural damage. This phenomenon can be caused by various factors, including soil type, water content, and human activities. Understanding the soilscape of a property is crucial in assessing the risk of subsidence and making informed decisions.

The Importance of Soilscapes in Property Assessment

Soilscapes, or the different types of soil found in a particular area, play a significant role in determining the stability of the ground beneath a property. Each soil type has unique characteristics that can influence its behaviour under different conditions. By understanding these characteristics, property buyers and investors can better assess the risk of subsidence and take appropriate measures to mitigate it.

Types of Soil and Their Impact on Subsidence Risk

Clay Soils

Clay soils are known for their high plasticity and ability to retain water. They can expand and contract significantly with changes in moisture content, making them particularly prone to causing subsidence.

Characteristics of Clay Soils:

• High Plasticity: Clay soils can change shape and volume when wet or dry.
• Water Retention: They can hold a large amount of water, leading to swelling.
• Shrink-Swell Potential: The soil can shrink when dry and swell when wet, causing ground movement.

Impact on Subsidence Risk:

• Seasonal Movement: Clay soils can cause seasonal ground movement, leading to cracks in buildings.
• Tree Influence: Trees and shrubs can exacerbate the problem by extracting water from the soil, causing it to shrink.

Sandy Soils

Sandy soils are composed of larger particles and have good drainage properties. They are less prone to significant volume changes with moisture variations but can still pose subsidence risks under certain conditions.

Characteristics of Sandy Soils:

• Good Drainage: Sandy soils allow water to pass through easily, reducing the risk of waterlogging.
• Low Cohesion: The particles do not stick together well, making the soil less stable.

Impact on Subsidence Risk:

• Erosion: Sandy soils can be prone to erosion, which can undermine the foundations of buildings.
• Compaction: They may compact under heavy loads, leading to settlement issues.

Silt Soils

Silt soils have smaller particles than sandy soils and can retain more water. They are more cohesive than sandy soils but less so than clay soils.

Characteristics of Silt Soils:

• Moderate Water Retention: Silt soils can hold water but not as much as clay soils.
• Smooth Texture: The soil has a smooth, floury texture when dry.

Impact on Subsidence Risk:

• Moderate Shrink-Swell Potential: Silt soils can expand and contract with moisture changes, though not as dramatically as clay soils.
• Erosion Risk: They can be prone to erosion, especially when exposed to water flow.

Loam Soils

Loam soils are a balanced mixture of sand, silt, and clay. They are considered ideal for agriculture due to their fertility and good drainage properties.

Characteristics of Loam Soils:

• Balanced Composition: Loam soils have a mix of sand, silt, and clay, providing good structure and drainage.
• Fertility: They are rich in nutrients and organic matter.

Impact on Subsidence Risk:

• Stable Structure: Loam soils are generally stable and less prone to significant volume changes.
• Moderate Risk: While they can still pose a subsidence risk, it is typically lower than that of pure clay or sandy soils.

Peat Soils

Peat soils are composed of decomposed organic matter and are found in wetland areas. They have high water content and can be very compressible.

Characteristics of Peat Soils:

• High Organic Content: Peat soils are rich in organic matter.
• Waterlogged: They can hold a large amount of water, making them spongy and compressible.

Impact on Subsidence Risk:

• High Compressibility: Peat soils can compress significantly under load, leading to settlement issues.
• Decomposition: As the organic matter decomposes, the soil can lose volume, causing subsidence.

Assessing Subsidence Risk: A Step-by-Step Guide

Step 1: Conduct a Soil Survey

The first step in assessing subsidence risk is to conduct a thorough soil survey. This involves collecting soil samples from various depths and locations around the property. A professional soil surveyor can analyse these samples to determine the soil type and its characteristics.

Step 2: Analyse Soil Properties

Once the soil samples have been collected, they need to be analysed to determine their properties. Key properties to consider include:
– Plasticity Index: Indicates the soil’s ability to expand and contract.
– Moisture Content: Determines how much water the soil can hold.
– Shear Strength: Measures the soil’s resistance to shear stress.
– Permeability: Indicates how easily water can pass through the soil.

Step 3: Evaluate Environmental Factors

Environmental factors can significantly influence the behaviour of soil and the risk of subsidence. Key factors to consider include:
– Climate: Seasonal variations in temperature and rainfall can affect soil moisture content.
– Vegetation: Trees and shrubs can extract water from the soil, causing it to shrink.
– Water Table: The level of the water table can influence soil moisture content and stability.

Step 4: Assess Building Foundations

The type and condition of a building’s foundations can also impact its susceptibility to subsidence. Key aspects to evaluate include:
– Foundation Depth: Deeper foundations are generally more stable and less prone to subsidence.
– Foundation Type: Different types of foundations (e.g., strip, raft, pile) have varying levels of resistance to ground movement.
– Foundation Condition: Cracks or damage to the foundations can indicate existing subsidence issues.

Step 5: Monitor Ground Movement

Monitoring ground movement over time can provide valuable insights into the risk of subsidence. This can be done using various techniques, such as:
– Levelling Surveys: Measure changes in ground level over time.
– Inclinometers: Measure the angle of slope movement.
– Settlement Plates: Track vertical ground movement.

Step 6: Implement Mitigation Measures

If a subsidence risk is identified, appropriate mitigation measures should be implemented to reduce the risk. These may include:
– Soil Stabilisation: Techniques such as soil compaction, grouting, or the use of geotextiles can improve soil stability.
– Drainage Improvements: Installing proper drainage systems can help manage soil moisture content and reduce the risk of subsidence.
– Foundation Reinforcement: Strengthening or underpinning existing foundations can provide additional support and stability.

The Role of Professional Surveyors

Professional surveyors play a crucial role in assessing subsidence risk and providing expert advice to property buyers and investors. They have the knowledge and experience to conduct thorough soil surveys, analyse soil properties, and recommend appropriate mitigation measures. Engaging a professional surveyor can provide peace of mind and ensure that potential subsidence risks are identified and addressed.

Case Study: The Impact of Soilscapes on Subsidence Risk

To illustrate the importance of understanding soilscapes, let’s consider a hypothetical scenario involving two properties in different soilscapes.

Property A: Located on Clay Soil

Property A is situated in an area with predominantly clay soil. The soil survey reveals a high plasticity index and significant seasonal moisture variations. The presence of large trees near the property further exacerbates the risk of subsidence.

Assessment and Mitigation:

• Soil Survey: The soil survey confirms the presence of clay soil with high shrink-swell potential.
• Environmental Factors: The large trees are identified as a contributing factor to soil moisture variations.
• Foundation Assessment: The foundations are found to be shallow and showing signs of cracking.
• Mitigation Measures: Recommendations include removing or managing the trees, improving drainage, and underpinning the foundations to provide additional support.

Property B: Located on Sandy Soil

Property B is located in an area with predominantly sandy soil. The soil survey indicates good drainage properties but a risk of erosion and compaction under heavy loads.

Assessment and Mitigation:

• Soil Survey: The soil survey confirms the presence of sandy soil with good drainage but low cohesion.
• Environmental Factors: The property is situated on a slight slope, increasing the risk of erosion.
• Foundation Assessment: The foundations are found to be in good condition but may require reinforcement to prevent settlement.
• Mitigation Measures: Recommendations include installing erosion control measures, such as retaining walls and ground cover, and reinforcing the foundations to prevent settlement.

Conclusion

Understanding the soilscape of a property is essential in assessing the risk of subsidence and making informed decisions. Different soil types have unique characteristics that can influence their behaviour under various conditions. By conducting thorough soil surveys, analysing soil properties, and evaluating environmental factors, property buyers and investors can better assess the risk of subsidence and implement appropriate mitigation measures.

At Flettons, we’re committed to safeguarding your investment. When considering a property purchase, trust our seasoned expertise to reveal any hidden threats. For a thorough building survey, get your instant quote through our quote calculator or reach out directly at 0203 691 0451. Your home’s safety is our top priority.

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